This invention relates to diffract gratings, and finds particular, but not necessarily exclusive, application in the field of optical transmission systems.
The channel capacity of an optical fibre transmission system can be greatly increased by using wavelength multiplexing in which many different signals are transmitted simultaneously along one fibre at different wavelengths. To achieve this, a satisfactory means must be provided at the transmitter end of a fibre link for combining the different optical wavelengths (multiplexing) and, at the far end of the fibre link, for separating them (demultiplexing). Multiplexing and demultiplexing may be achieved by means of channel dropping filters but a disadvantage of this approach, particularly where a large number of channels is involved, is that a separate filter is required for each channel at each end of the link. At optical frequencies an alternative to the channel dropping filter is the diffraction grating, one of whose advantages is that a single grating can be used for multiplexing an appreciable number of channels, and a second grating for demultiplexing them all. If the optical link is provided by an optical fibre waveguide, then the grating may convenently be formed in a slab waveguide comprising a layer of dielectric material bounded by material of lower refractive index. Such a slab constitutes a one-dimensional waveguide, providing a waveguiding effect normal to the plane of the slab, but, unlike conventional two-dimensional waveguides, does not confine light propagating in the slab to any particular path within the slab.
A reflective type one-dimensional diffraction grating can be made in a slab waveguide by providing one edge of the waveguide with an array of reflecting facets. In the case of a conventional two-dimensional reflection-type diffraction grating the light is propagating in air when it is incident upon the grating facets, and these facets are typically made of metal or are metallized in order to make them sufficiently reflecting. In the single dimensional slab waveguide diffraction grating metallisation is not required because the light is propagating in the material of the slab waveguide when it is incident upon the grating facets, and hence, by using a sufficiently large angle of incidence, advantage can be taken of the phenomenon of total internal reflection. The advantage of making use of total internal reflection is that it does not exhibit the absorption characteristic of reflection at a metal surface, and nor is the amplitude of the reflection sensitive to the plane of polarisation of the incident light. For a slab waveguide made of semiconductive material, such as silicon or InGaAsP, the relatively high refractive index provides total internal reflection at a semiconductor/air interface for angles of incidence greater than about 20.degree..
For a wavelength multiplexed optical transmission system the grating must provide adequate resolution to discriminate between adjacent channels, and it must also avoid introducing significant additional optical loss into the transmission path. In the case of a slab waveguide grating it is also beneficial to design it in such a way as to minimize the optical path length between the object, the grating and the diffracted image because this also minimizes the effects of any non-uniformity in the slab waveguide that produces distortion of the optical wavefront. Path lengths in the collimating sections of such gratings tend to be proportional to the projected length of the grating in the direction normal to the direction of illumination, whereas the resolution of the grating is approximately proportional to its projected length in the direction parallel with the direction of illumination. Thus to reduce the path length while still achieving adequate resolution generally involves orienting the grating so that light is incident upon it at a considerable angle of incidence, typically 45.degree. or more. This requirement, and the need to blaze the grating so that the input and output beams are inclined to each other by an angle at least twice as great as the critical angle, can be met in a format of one-dimensional diffraction grating that is fabricated in slab waveguide and is derived from the format of a conventional reflection type two-dimensional diffraction grating, and such a format has been described by S. Valette et al. in a paper entitled "New Integrated Optical Multiplexer Demultiplexer Realised on a Silicon Substrate", Proceedings of the 4th European Conference on Integrated Optics (ECIO'87) 11th-13th Nov '87 (Ed. CDW Wilkinson, J. Lamb--Publ. SETG).
This format is illustrated in FIG. 1 of the accompanying drawings, and from this figure it can be seen that the combination of a considerable angle of inclination of the grating with the need to separate the input and output beams by at least twice the critical angle detracts considerably from the efficiency of the grating. FIG. 1 is a planar view of a slab waveguide 10 provided with an aperture 11, extending with perpendicular walls, through the whole thickness of the waveguide.
The wall which forms the grating is serrated with a pitch `d` being composed of a set of facets 12 separated by the members of a second set of facets 13. Light is arranged to be incident upon the grating at an angle `I` and the grating is blazed so that the reflecting facets of the grating facets 12, are inclined at an angle .theta. to the direction of the grating. The incident light strikes each reflecting facet at an angle of incidence .phi. (where I+.phi.=.theta.) so that the diffracted light which is specularly reflected by each facet is inclined to an angle 2.phi. to the incident light. If it were not for the need to exceed the critical angle for total internal reflection, .phi. could be zero, in which case the grating would be the slab waveguide equivalent of an echelon grating which in principle can approach 100% efficiency because every part of the incident beam is reflected by one of the reflecting facets. In this instance however .phi. cannot be zero, and the result is that a proportion of the incident light is lost through being scattered or diffracted into unwanted orders of being incidentally directly upon the facets 13 or, if those facets are differently oriented, through being incident upon them after reflection in the facets 12. From geometrical considerations it can be seen that if .theta.=45.degree. and .phi.=20.degree. the loss in efficiency is over 6 dB. (If the direction of the light is reversed the efficiency loss through this particular form of unsatisfactory illumination occasioned because the facets 13 are in the shadow of their adjacent facets 12 is avoided, but the place of this loss mechanism is taken by another, namely the diffraction loss into unwanted orders resulting from the fact that the reflected light is divided into separated bands as it leaves the grating.)